1. Field of the Invention
The present invention relates to a brushless DC motor, and more particularly to a brushless DC motor which does not employ a special position detecting means to obtain a rotating position signal for switching an excited phase during a normal rotation.
2. Description of the Prior Art
Brushless DC motors have been used in video tape recorders (VTRs) for driving the rotary head. In the conventional brushless DC motor for driving the rotary head of VTR, since the excited phase of stator coils is switched according to the rotation of the rotor, a plurality of special position detecting means such as the Hall elements are provided to detect a rotor position, and furthermore, another position detecting means is provided to detect a standard rotating position of the rotor so that the rotary head mounted on the rotor can scan specified positions on a magnetic tape. However, use of such position detecting means is one of the obstacles to reduce cost and size of the motor.
Therefore, some brushless DC motors which do not use the position detecting means have been proposed. In these brushless DC motors, some processes are provided to obtain the rotating position signal for switching the excited phase of the stator coils by detecting the counter electromotive forces generated in the stator coils. Since the counter electromotive force is used to obtain the rotating position signal, the rotating position signal cannot be obtained when the rotor is not rotating. Therefore, when starting the motor, a specific stator coil is energized for positioning the rotor thereby to detect a starting position. This technique is disclosed, for example, in the Japanese Laid-Open Patent Application No. 55-160980.
However, in such brushless DC motors, when positioning the rotor at starting, certain time is required until the permanent magnet rotor comes to a standstill at a desired position because the rotor vibrates about the desired position. The time required for the permanent magnet rotor to become stationary varies considerably with the load or the inertia of the rotor, and is fairly long particularly when the inertia of the rotor is large. Furthermore, if the rotor is in a position apart by 180.degree. from the desired position where the rotor is to be positioned, the rotor cannot rotate unless no disturbance is caused, resulting in failure of starting.